Tires

[Defrost]

I just replaced both the tires and rims on my '07 Mazda 5. It came with the smaller-style tires, which I realize are very popular right now, but I didn't like them. I chose to get smaller rims in order to fit a much larger tire than what the vehicle came with, and I noticed at work today that they were very close in size to my co-worker's Saturn Vue.

I am really completely amazed at the difference. MUCH better traction, noticeably smoother ride. I'd been told this would be the case, but I didn't think it would be this obvious. The more I drive with the new tires the more pissed I get about the old ones!

My question now is WHY are the small tires so popular? Is it purely cosmetic, or are there actually driving conditions where smaller tires are better? Aside from the traction issue, is there any other risk with smaller tires?

[joolsplus3]

I have no idea, I just buy what the Costco guy tells me to...but... Does changing the tire size have any effect on your speedometer? I knew a guy with a lowered truck and larger rims and he thought he was driving 90 everywhere because his speedometer wasn't set to match the tires. Anyway, just curious...Glad your new tires are better!

[jpzec]

Hi Debbie - Would you mind giving details of your new tires?? We have had our Mazda 5 for just at a year and I've detested those puny little tires the whole time! We've talked about replacing them, but just haven't gotten around to it!

Thanks -- Jamie
(Frequent lurker/former tech)

[jen_nah]

Yes, Changing tire size can change the speedo on a vehicle to read fast or slow depending on which way you go tire size and how much bigger/smaller you went.

I didn't know smaller was popular. Here it's big rims & big tires that are the cool thing.

I just bought new tires for my Tahoe last month. And boy was I missing my smooth ride. What a world of differance new tires can make on how the vehicle rides. I replaced mine with the same ones that came on my Tahoe. Also b/c I was cheap and knew we were getting rid of my Tahoe ath the end of the year so I didn't want to put a lot of money into new tires to only get a year out of them.

[southpawboston]

okay, here goes my tire shpeal:

there are THREE aspects to a tire size:

1) rim diameter. this is the diameter of the wheel rim, in inches (15", 16", 17", etc)
2) tread width, in millimeters (195, 205, 215, etc). this is what makes a tire look wide or narrow
3) aspect ratio. this is the most confusing component. it is the distance from the rim to the tread, or the "height", or the "side wall", and it is expressed as a percentage of the width. for example, the mazda5 stock tire's aspect ratio is 50, and the stock width is 205. that means that the side wall is 50% of 205 mm, or 125 mm.

when you see the tire size on the side of the tire, it looks like this: 205/50 R17. 205 is treadwidth, 50 is aspect ratio, and R17 means 17" rim, radial tire.

what does this all mean?

for comfort and smoth ride, you want a tall side wall (high aspect ratio). the higher the side wall, the more cushioning there is between the road and the rim. the tradeoff is less road feel, less precise steering, and overall less control of your car.

for sharp, crisp handling, you want a short side wall (low aspect ratio). the tradeoff is comfort, as bumps well be felt more. there is simply less cushioning between the road and the rim.

traction does NOT increase by increasing aspect ratio, except for snow tires. that is a whole other discussion.

changing the width of the tire without changing the aspect ratio will change the overall circumference of the tire, which will cause your speedometer to read incorrectly. generally speaking, for every 10mm wider in tire size, the aspect ratio must go down about 5% to keep the overall tire circumference the same and keep your speedometer accurate.

so, in defrost's case, if she wanted a softer riding tire without changing her rims, she would go from a 205/50R17 tire to a 225/50R17 or 225/60R17. this will cause her speedometer to read slower than she is actually going, because the larger circumference tire require fewer rotations to cover the same distance.

alternatively, she could go with a smaller rim (16") and 215/65R16 tires. this will gain her a little bit of sidewall height, but keep the overall tread circumference the same. speedometer is unaffected. this is what i did for my winter tire/rim setup.

since mazda builds their cars to be driver-oriented and sporty, they choose to put on sport tires with low aspect ratios.

EDIT: i made up my own little tire size calculator with which you can input the three tire size numbers and compare your speedometer error against the stock tire size. i used that to determine what size winter tire to put on my car to maximize tire side wall height without afecting my speedometer. just holler if you want me to compare two or more tire sizes. :thumbsup:

[Defrost]

Thanks SPB! I was kinda hoping you'd chime in.

Yes, I chose taller sidewalls. I had to get smaller rims for that. I think the tread width is the same. Let's see if I can translate the receipts now that SPB has explained it...

Okay, like SPB said, the old tires were 205/50R17.
The new ones are "P205/55R-16." So, the tread width is the same, the aspect ratio went from 50 to 55, and the rims from 17" to 16". Right?

And the traction is vastly improved. The more I drive on the new tires the more I love them. I used to have trouble with spinning out on even slightly wet pavement, and with the killing the engine because it's a manual transmission, and I had trouble giving it enough gas to keep the engine running without giving it so much that I was spinning out.

Maybe it's because the new tires are all-seasons and the old ones weren't, rather than the size change?

Anyway, I'm really glad SPB offered so much info. All I could have told you was what I told the guy at the tire place, and it took him about fifteen minutes to finally figure out what I wanted! (It involved a lot of "No, no don't want studded tires." "I don't care if it's ugly." and "I am not interested in COOL tires!" :p )

[jpzec]

Thanks, guys! Now to save the bucks and take the plunge!

Jamie